The present invention relates generally to electric or hybrid electric vehicle propulsion systems. More specifically, the present invention relates to the design of electric traction motors or machines for use in electric or hybrid vehicles.
In today""s automotive market, there exists a variety of electric propulsion or drive technologies used to power vehicles. The technologies include electric traction motors such as DC motors, AC induction motors, switched reluctance motors, synchronous reluctance motors, brushless DC motors and corresponding power electronics. Brushless DC motors are of particular interest for use as traction motors in an electric vehicle because of their superior performance characteristics, as compared to DC motors and AC induction motors. Brushless DC motors typically operate with a permanent magnet rotor. A permanent magnet rotor may be configured as a surface mount or interior or buried permanent magnet rotor. An interior permanent magnet (IPM) motor or machine has performance attributes, when compared to DC motors and AC induction motors, that include relatively high efficiency, relatively high torque, relatively high power densities, and a long constant power operating range which make an IPM machine attractive for vehicle propulsion applications.
Permanent magnets buried inside a rotor for a brushless DC motor exhibit high reluctance directly along the magnetic axis or the d-axis due to the low permeability of the permanent magnets, while along the q-axis, between the magnetic poles or magnet barriers of an IPM rotor, there exists no magnetic barrier and reluctivity to magnetic flux is very low. This variation of the reluctance around the rotor creates saliency in the rotor structure of an IPM machine. Therefore, the IPM rotors have reluctance torque in addition to the permanent magnet torque generated by the magnets buried inside the rotor. Reluctance in the d-axis can be created by one magnet such as found in a single barrier rotor design.
A single magnet of the one barrier rotor design can also be split into several layers creating a multi-barrier design. The multi-barrier design reduces leakage and improves the rotor saliency. Accordingly, motors having multi-barrier rotors have numerous performance advantages over a single barrier rotor design, including relatively high overall efficiency, extended high speed constant power operating range, and improved power factor. Improved saliency of the multi-barrier rotor helps to lower the amount of magnets or magnetic material in an IPM machine, as compared to a single barrier IPM machine or surface mounted permanent magnet machine, by reducing dependency on magnetic torque. The amount of magnetic material needed to generate a specific torque and wattage rating depends on the level of saliency of the rotor. The higher the rotor saliency, the lower the amount of magnetic material usage for the same overall machine performance. Electric motors having a multi-barrier rotor design, as compared to single barrier design, generate higher rotor saliency.
Magnets in an IPM machine can be pre-magnetized and then inserted inside the rotor. This magnet insertion is a complex and relatively costly step that adds manufacturing steps to the assembly of the IPM machine.
Post-magnetization of inserted magnetic material is possible if the magnets are inserted near the rotor surface. For post-magnetization, magnetic material may be preformed outside of the rotor, inserted into the rotor, and then magnetized. This is usually the case with sintered magnets, which require a certain orientation. A further type of magnetic material used that may be used in an IPM rotor is bonded magnets, which are usually mixed with a plastic, such as PPS, and may also be preformed outside of the rotor and then inserted into the rotor. However, generally bonded magnetic material is injected into the rotor cavities under high temperature and pressure.
Electric motors having multi-layer buried magnets in their rotors, as shown in FIG. 2, exhibit excellent performance characteristics for vehicle propulsion application. The problems associated with post-magnetizing such a rotor geometry would result in a large amount of magnetic material buried deep within the rotor that may only partially magnetize or not magnetize at all, resulting in a waste of material. Post-magnetization works efficiently for magnetic material buried or located near the surface of the rotor. For magnetic material buried relatively deep in the rotor, post-magnetization is difficult due to the weakening of the magnetizing field.
The present invention includes a method and apparatus for the design of an IPM machine rotor. The present invention removes magnetic material from the regions of the rotor which cannot be effectively or strongly magnetized during the post-magnetization process and inserts magnetizing coils. The outer barriers of the rotor of FIG. 2 are relatively easy to magnetize. However, the middle section of the inner regions of the rotor may not be exposed to a magnetic field strong enough to fully magnetize these regions. In the present invention, magnetic material is removed from these middle section areas and magnetizing coils are inserted in the empty areas for the magnetization process. The inserted magnetizing coils will enhance the magnetizing field produced by the stator or other magnetizing fixture, thus improving the rotor magnetization. Keeping the middle section areas void of any magnetic material does not change the rotor saliency or the reluctance torque, provided that the remaining areas are filled with magnet material that is fully magnetized. Specifically, the bridges between voids and filled areas are saturated by the magnetic material so as to ensure saliency. The magnetizing coils that are inserted in the void areas for the magnetizing process will enhance the field in this region and help magnetize the magnetic material that may not be fully magnetized by the stator fixture. The magnetizing coils in the preferred embodiment are removed from the rotor following the post magnetization process.